The present invention relates to a weather resistant steel, and more particularly, to steel having excellent seashore weather resistance, capable of being used in a salty environment such as a seashore district and the like where a large amount of salt is present. The seashore weather resistance described here is the weather resistance of steel when it is used in a salty atmosphere in a seashore district.
Weather resistant steel whose corrosion resistance in the atmosphere is improved by adding alloy elements such as Cu, Cr, Ni, etc. to it has been widely used in structures such as bridges and the like. In weather resistant steel, rusts called stable rusts, which oxygen and water acting as a cause of rusts cannot easily penetrate, are formed on the steel in several years and thereafter the corrosion of the steel is suppressed by the stable rusts. As a result, the weather resistant steel is a less expensive highly corrosion resistant material which can be used effectively because it need not be coated with a rust-preventing paint.
In contrast, according to a guide for application of weather resistant steel recently made public by Ministry of Construction (xe2x80x9cReport of Joint Researches on the Application of Weather Resistant Steel to Bridgesxe2x80x9d (XX), 1993.3, published by Public Works Research Institute of Ministry of Construction, The Kozai Club and Japan Association of Steel Bridge Construction), conventional weather resistant steel (JIS G 3114: weather resistant hot rolled steel used for welded structure) cannot be used without painting in a region where the amount of airborne salt is not less than 0.05/dm2/day, that is, in a seashore district.
Therefore, ordinary steel is coated with phthalic resin, chlorinated rubber, tar epoxy resin, etc. in an environment such as the seashore district and the like where a large amount of salt is present, to cope with this regulation. Bridges constructed in the seashore district near the mouth of a river are severely corroded and it is often required to repaint them. However, many of them are long bridges where a repaint job is often difficult. Such being the case, there is still a request for steel which can be used without painting.
Further, the amount of airborne salt is greatly different depending upon the location of seashore districts.
Since an increase in the amount of airborne salt more severely corrodes steel, weather resistant steel corresponding to an amount of airborne salt is necessary from the view point of corrosion resistance and cost efficiency.
Further, the corrosion environment of steel used in bridges is not always the same depending upon the locations where the steel is used. For example, a portion outside a girder is exposed to rain, dew-condensed water and sunshine, whereas a portion inside the girder is exposed only to the dew-condensed water and is not exposed to rain. In general, it is said that the portion inside the girder is more violently corroded than the portion outside the girder in the environment where a larger amount of airborne salt is present.
To cope with the above problem, Japanese Unexamined Patent Publication No. 6-136557, for example, proposes a steel surface treatment method of applying a chromium sulfate aqueous solution or a copper sulfate aqueous solution to steel and further coating the steel with an organic resin film after water is dried. Further, Japanese Unexamined Patent Publication No. 8-13158 proposes a steel surface treatment method of applying an aqueous water solution containing aluminum ions to steel and further forming an organic resin film on the steel after water is dried. However, such process is complicated and a surface treatment agent to be used is expensive. These problems still remain in the technologies disclosed in Japanese Unexamined Patent Publication No. 6-136557 and Japanese Unexamined Patent Publication No. 8-13158, although stable rusts can be formed thereby in a short period of time. Thus, it has been desired to develop weather resistant steel which needs no surface treatment.
As to this point, Japanese Unexamined Patent Publication No. 63-255341 proposes a corrosion resistive steel plate for welded structure excellent in salt damage resistance. The steel plate contains P:0.04-0.15 wt %, Cu: 0.1-0.5 wt %, Cr: 3-10 wt %, and Al: 0.02-1.0 wt %, and it is said that the steel plate can be used without painting in a corrosive environment in which sea salt particles are concerned. However, since the steel plate contains a large amount of P, it has a problem that the toughness and weldability thereof are greatly lowered.
Further, Japanese Unexamined Patent Publication No. 3-158436 proposes seashore weather resistant structural steel. It is said that the steel exhibits excellent weather resistance in a seashore district by adjusting the contents of Mn, Cu, Cr, Ni and Mo without adding P in a large amount and painting thereof can be omitted.
However, a problem still remains in that the toughness and weldability of the steel are lowered because it contains Cr in a large amount.
In view of the above problems, an object of the present invention is to provide weather resistant steel which does not need painting, surface treatment or the like, and exhibits excellent seashore weather resistance corresponding to an amount of airborne salt even in such an environment as a seashore district and the like, where salt is present in a large amount and no rain cover is available.
A rust layer, which is formed at the initial stage of rusting weather resistant steel, is mainly composed of xcex3-FeOOH and Fe3O4. When the rust layer is subjected to a corrosion reaction by being repeatedly dampened and dried for several years, the contents of xcex3-FeOOH and Fe3O4 are reduced from the rust layer and it is mainly composed of amorphous rusts when observed using X-rays.
The ratio of the amorphous rusts in the rust layer formed on weather resistant steel exposed in a seashore district is smaller than that of weather resistant steel exposed in a non-seashore district. Accordingly, the ratio of amorphous rusts in a rust layer must be increased to provide steel with excellent seashore weather resistance.
As a result of diligent study, the inventors found that the ratio of the amorphous rust could be satisfactorily increased by reducing the C content of the components of steel. FIG. 1 shows the effect of the C content in steel, when the C content was varied, on the existing ratio (wt %) of amorphous rust in the rust layer formed after the steel was exposed to the salty environment where 0.8 mg/dm2/day of airborne salt was present when it was measured by a method to be described later. The steel contained, by wt %, a basic component of 0.3% Sixe2x80x941.0% Mnxe2x80x942.7% Nixe2x80x940.4% Cuxe2x80x940.0018% B. The existing ratio of the amorphous rust was increased by a decrease of the C content. Then, the existing ratio of the amorphous rust exhibited a high value of not less than 55% when the C content was 0.025% or less. FIG. 2 shows the relationship between the existing ratio of the amorphous rusts and the reduced amount of a sheet thickness at the time. It can be understood from FIG. 2 that the existing ratio of the amorphous rusts is large, the reduced amount of sheet thickness is made small. It is needless to say that the larger existing ratio of the amorphous rusts decreases the reducing speed of sheet thickness after the steel is exposed for one year.
The existing ratio of the amorphous rusts to the crystalline rusts is a value obtained by measuring the content of crystalline rusts (weight) in a rust layer formed using X-ray diffraction, determining the weight of amorphous rusts by subtracting the weight of the crystalline rusts from the total weight of the rusts and dividing the weight of the amorphous rusts by the total weight of the rusts.
The present invention is based on the above knowledge.
That is, the present invention is a weather resistant steel which comprises, by wt %, C: 0.001-0.025%, Si: not more than 0.60%, Mn: 0.10-3.00%, P: 0.005-0.030%, S: not more than 0.01%, Al: not more than 0.10%, Cu: 0.1-1.5%, Ni: 0.1-6.0%, B: 0.0001-0.0050%, and the balance being Fe and inevitable impurities.
Further, we have found that weather resistance can be further improved by adjusting the content of B and the contents of one or more of P, Cu, Ni and Mo according to a formula (1) in relation to the content of salt.
(11P+4.0Cu+3.1Ni+2.6Mo)/(1xe2x88x920.1(10000 B)0.35)xe2x89xa71+13Xxe2x80x83xe2x80x83(1)
(where, P, Cu, Ni, Mo and B: contents of respective elements (wt %), and X: content of airborne salt (mg/dm2/day)).
The value X is measured by a gauze method regulated by JIS Z 2381.
Further, in the present invention, one or two or more of Nb: 0.005-0.20%, Ti: 0.005-0.20 and V: 0.005-0.20% may be further present by wt %, in addition to the above components. Furthermore, in the present invention, REM: not more than 0.02% may be further present, in addition to the above components.